scripts: Add slicesnoop script

[hodgesds]

Add slicesnoop.bt bpftrace script that allows the introspection of slice durations aggregated by DSQs.

Example of scx_layered tracing the distribution of high fallback dsq (1024) slice duration:

$ sudo ./slicesnoop.bt 0 1024
@dsq_slice[1024]: 
[512K, 1M)           528 |@@@@@@@@@@@@@                                       |
[1M, 2M)               0 |                                                    |
[2M, 4M)               0 |                                                    |
[4M, 8M)               0 |                                                    |
[8M, 16M)              0 |                                                    |
[16M, 32M)          2051 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

@dsq_slice_avg[1024]: 16069174

[likewhatevs]

LGTM

[etsal]

Looks good!